1. Field of the Invention
The present invention relates to a process for the production of diaryl carbonates, such as diphenyl carbonate. More particularly, the invention relates to a process for the production of diaryl carbonates wherein a series of fixed bed reactors withdraw feed from a single distillation column at different separation stages and return the product to the column. The reactors may be operated at different conditions than those of the distillation column to optimize production of diaryl carbonates.
2. Related Art
Diphenyl carbonate is a valuable intermediate for the production of polycarbonate. In current commercial practice, diphenyl carbonate is produced by transesterification with an alcohol followed by disproportionation using dimethyl carbonate (DMC) and phenol. Such processes are disclosed in U.S. Pat. Nos. 5,210,268; 6,197,918; and 6,262,210. The typical reaction is:

The unconverted dimethyl carbonate is recovered as a mixture with the coproduct methanol. Dimethyl carbonate is separated from the mixture and recycled. However, dimethyl carbonate and methanol form an azeotrope. Therefore, the recovery of dimethyl carbonate in the mixture involves breaking the azeotropic mixture which is energy consuming and expensive.
Heretofore higher conversions to alkyl phenyl carbonate and thence to diphenyl carbonate have been achieved by carrying out the reaction in two steps each of which involves continuous removal of the lighter components from the reacting mixtures. In the first step the alkyl alcohol is the lighter component, and in the second step dialkyl carbonate is the lighter component. This has normally been achieved by some form of reactive distillation wherein the reaction in the presence of the catalyst is carried out with simultaneous separation of components by distillation. Such processes can be efficient provided the reaction rates are adequate in the temperature range set by the vapor-liquid equilibria of the distilling system. Pressure and reflux ratio can be adjusted to effect the reaction zone temperature profile and/or reaction volume can be increased but such changes can increase the cost of the process.
U.S. Pat. No. 6,093,842 addressed this problem by using reactive distillation and an entraining agent that did not form an azeotrope with the dialkyl carbonate or the alkyl alcohol. The patentee states, “While the dialkyl carbonate used as a starting material in the method of the invention may in theory be any dialkyl carbonate, it will be appreciated that in practice it will be preferred to use a dialkyl carbonate with a small alkyl group, such as, for example dimethyl carbonate.” However, the methanol is the cause of the azeotrope problem and the solution merely changes the manner of breaking the azeotrope. Also producing the dimethyl carbonate in the first place involves the same problem, i.e., breaking the azeotropic mixture to separate the product, dimethyl carbonate, from the product stream.
The present invention avoids the formation of azeotropes in the transesterification process.